FIGURE 1. MX- 28 out
of the box, with
horn, thrust washer,
hardware, and cable.

main body of the servo is identical except for the electrical
connector is three-pin instead of four-pin. The version of the
RX- 28 that I used in the hexapod was purchased a few years
ago, so the servo horn was a bit different style. The
threaded servo horn holes have actually been drilled on a
slightly different radius. The newer horn style has threaded
holes on a 16 mm diameter, whereas the older horn style
has holes on a 17 mm diameter. When upgrading, it is
important to make note of this as any brackets will need to
be redesigned or drilled out.

Fortunately, this issue is fairly minor compared to the
different communication bus. The serial control protocol is
still identical, however, Robotis switched the protocol from
the RS-485 bus to bidirectional half-duplex TTL
communication. This transition is nice in that it reduces the
overall wiring which will definitely be helpful when trying to
cram all the excess wire inside the hexapod body. I believe
the TTL bus also consumes a bit less power than the RS-485
bus. The USB2Dynamixel supports the TTL bus as well, and
now the MX- 28 can be daisy-chained directly to the AX- 12
or AX- 18 actuators.

Unfortunately, the half-duplex TTL bus is not as well
supported outside of Robotis. Before, it was possible to get
any FTDI based USB to RS-485 adapter and very quickly run
some Linux code and begin communicating. Without the
USB2Dynamixel, a method of tristate buffers (like chips

74HC125 or 74HC126) must be implemented to
communicate through a standard UART or FTDI chip. It is by
no means a difficult circuit to build; it is just not quite as
plug and play as before. It’s nice to see that Robotis appears
to be making a move to have all their motors use the same
protocol. Anyone already set up with the AX-series will
definitely be happy with the new transition. I would think
that when the MX- 64 and MX-106 come out they may also
have the TTL bus.

Inside the MX- 28

Opening up both the MX- 28 and RX- 28, we can see a
bit of what is going on internally. In Figure 3, the RX- 28 is
on the left, showing the L6201 H-bridge. On the right is the
MX- 28, showing the 72 MHz STM32F103C8T6 ARM Cortex
M3 processor — which is quite a step up from the ATMega8
in the RX- 28. Both motors include a spot for a programming
header, enabling any hacker to quickly load up their own
code. Figure 4 shows the opposite side of the MX- 28
board. Here, the 74HC126 tristate buffer is visible, as well as
the AS5045 magnetic potentiometer. Still in the body of the
servo is the magnet attached to the output shaft. Clearly,
the old slot for the potentiometer used in the RX- 28 still
exists, meaning that the body is most likely interchangeable
with the RX- 28. Figure 5 shows the gearbox of the MX- 28.

The magnetic sensing is what puts the M in MX- 28. This
is the most exciting part for implementing the motor in the
hexapod as it will never wear out since there is no
mechanical contact. The AS5045 uses a set of Hall-effect
sensors to determine the orientation of the magnet. The
previous continuous turn potentiometers eventually became
worn after running the hexapod for an inordinate amount
of time. The magnetic sensing also extends the angle
control range to a full 360 degrees (from 300) at a 12-bit
resolution (from 10-bit). This increases the precision from
.293 degrees to .0879 degrees. That is a total factor of 3. 33
times better.

The new ARM processor is able to bump up the
communication bus to include three newer baud rates at
2250000, 2500000, and 3000000. I will definitely be
implementing the newer baud rates which will make the
hexapod operate as a much smoother machine.